With rapid progress of the electronic techniques lately, the field of photoelectronics such as liquid crystal display elements, electroluminescent display elements and photoelectronic transfer elements for solar cells has been spreading steadily.
In such fields, photoelectronic elements have been supplied to various uses generally for such elements placed on a glass substrate having a transparent conductive layer.
Since, however, glass is problematic due to its insufficient mechanical strength, especially brittleness, this resulting in lowering of durability of elements and, to cope with it, substrates specially treated like those of tempered glass are now being often used. When such elements are incorporated into portable devices in particular, their weight is increased due to the large specific gravity of glass. Therefore, thinning of the glass substrate is required and substrates 0.4 mm or so in thickness are now feasible. However, its brittleness still remains and problems such as lowering the yield due to breaking in the process or lowering of impact strength of the elements remain unsolved.
Thus, strengthening and weight-saving of substrates are strongly desired and from the viewpoints of lightness, impact strength and interchangeability with glass, generally preferred are optically transparent (i.e. 80% or more in ray transmission and 5% or less in haze) plastic substrates having 0.2-0.5 mm thickness and having a reasonable rigidity.
With respect to the process temperature required for the formation of photoelectronic elements taken into consideration, however, a high heat resistance of not less than 180.degree. C., preferably not less than 200.degree. C. are required and, when application to liquid crystal elements is considered, a low retardation preferably of not more than 50 nm, more preferably, not more than 20 nm, is required. Meanwhile, when a plastic film or sheet is made, its molecules are subjected to orientation, and it is particularly difficult to obtain a film or sheet which is low in retardation by a melting method, especially when the material used is high in heat resistance. For example, when a material used is relatively low in heat resistance such as polycarbonates, it is even possible to obtain a sheet low in retardation of not more than 50 nm which is usable for the production of liquid crystal display elements, but no low-retardation sheet is obtainable from a material of not less than 180.degree. C. in glass transition temperature such as polyarylates.
Meanwhile, in the case of supernematic liquid crystal displays (STN-LCD), it is a general practice to use a high molecular film (phase difference film) with its retardation, which is represented by the product of birefringence and thickness, controlled to a specific value stuck to a glass substrate in order to improve its display quality. When the substrate is made of a plastic, features not obtained with a glass substrate such as imparting the phase difference function thereto for cost saving are expected. Thus, particularly desired is a plastic substrate which is heat-resistant with its retardation well controlled to be not less than 100 nm, preferably in a range of about 100-700 nm.
For that purpose, proposed is a method of using heat-resistant high molecular materials such as polyarylates, polysulfones, polyethersulfones, and polyetheretherketones made in film form by a solvent casting method and used in the form of the so-called plastic liquid crystal cells etc. (Japanese Laid-open Patent Publication No. 119321/'84, Japanese Laid-open Patent Publication No. 167208/'85, Japanese Laid-open Patent Publication No. 147721/'85, Japanese Patent Publication No. 41539/'86, U.S. Pat. No. 4,623,710), but such plastic substrates are filmy some 0.1 mm in thickness. These are, however, more flexible and less rigid than glass, hence cannot be used in common in processes in which glass substrates are used. Thus, it is not proper for the purpose of eliminating the defect of glass substrate by the use of any existing glass substrate process, and the development of a new process is required.
In order to improve the rigidity of such film, it is considered to increase the film thickness, but if the increase of the film thickness is intended by a solvent casting method, defects such as bubbling are likely to occur and, worse, marked lowering of producibility makes its application to commercial production difficult, the upper limit thus being about 200 .mu.m. When the increase of film thickness is intended by a melt-extruding method, optical isotropy is not only lost but, surface smoothness or external appearance is bad due to die lines formed during molding, consecutively, it is difficult to use such films as liquid crystal display substrates.
Meanwhile, the possibility of film-forming essentially low birefringent plastics such as polymethyl methacrylates and modified polyolefins or curing-type plastics such is crosslinked acryl resins or epoxy resins as disclosed in Japanese Laid-open Patent Publication No. 194501/'94 for this purpose. However, former lacks in heat resistance required for glass process, while the latter is problematic with its poor producibility and is not suitable for mass-production. It is also possible to make a sheet by laminating the aforementioned heat resistant optical film, but there is no adhesive excellent in both heat resistance and reliability, and a high treating temperature is required for heat lamination which causes deterioration in properties such as denaturation and tinting of resins.
Thus, there has been found to date no commercially usable materials having the rigidity and the heat resistance required for having interchangeability with the glass process, and also being excellent in impact strength as well as in optical properties such as high transparency and low retardation.
In view of such actual situation, the present invention provides a good heat-resistant optical plastic laminated sheet excellent in heat resistance and optical properties and having good mass-producibility, being thus well suited for use as photoelectronic elements.
After intensive studies for attaining the aforementioned objects, the present inventors discovered that a laminated structure comprising a layer having a high heat resistance and another layer excellent in physical properties at room temperature provides a good optical plastic laminated sheet having a high rigidity and excellent optical properties as well as high heat resistance, and arrived at the present invention.